Grain refining process



Dec; 13, 1966 v. PETRovlcH 'GRAIN REFINING PROCESS Filed Feb. 6, 1964XCMEmWW W xmbtmwm.

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INVENTOR. Wadi if' Paf/'ow' h United States Patent O 3,290,742 GRAINREFINING PROCESS Vladimir Petrovich, Midland, Mich., assignor to The DowChemical Company, Midland, Mich., a corporation of Delaware Filed Feb.6, 1964, Ser. No. 343,067 9 Claims. (Cl. 22-200.1)

This invention relates to grainrefining and more particularly relates toa novel method of grain refining a m-agnesium-base alloy containing zincand misch metal.

When casting magnesium alloys containing up to 1.0 percent of r-areearth metals and at least about 0.3 percent zinc, lfor example, the ASTMdesignated magnesiumbase alloy ZE10, which are usually cast at atemperature within the ran-ge lof from about 1300 to l350 F., columnarygrains and sO-called stringers, as defined below, tend to form in theouter portions commencing near the chilled surface and on in toward thecenter of the cast ingots or billets rather than equiaxed grains whichare more desirable. Such columnar structure is particularly undesirablein castings intended `for rolling in that it may cause the metal tocrack severely when lrolled. However, when equiaxed grains are presentthe rollability of the alloy is increased t-ogether with its strengthpotential.

The term columnar grains as used herein refers to a course structure ofparallel columns of lgrains having the long axis perpendicular to thecasting surface, whereas, the term stringers refers to a band at or nearthe casting surface comprising shorten columnar grains but, :like thecolumnar grains, not equiaxed in structure. Accordingly, the -tenmequiaxed Igrain refers to a grain structure in a magnesium-.base alloywherein the axis Iof substantially all the grains is substantially thesame length 'across 'any 4'given cross isection thereof. y

Various methods have been and are presently practiced to obtainpreferably a long lasting grain refinement effect in magnesiumbasealloys such as, for example, by adding metallic elements to the alloy asnucleating agents such as, for example, zirconium, iron, aluminum, andcarbon. The addition of such agents, however, to `grain refine said ZE10alloy in Vaddition to being economically undesirable, has been found tobe unsatisfactory in that by adding zirconium, for example, the`cleanliness of the molten alloy deteriorates, that is, the num-ber ofoxide particles and flux inclusions increases in castings. The additionof iron onV the other hand rdoes not normally result in a lasting grainrefinement at high alloying and casting temperatures. In addi-tion, ironmay create corrosion problems in the alloy. Furthermore, carbontreatments are not effective on alloys, such as ZE10, which do notcontain aluminum. In view of the above difficulties,

therefore, it would be highly desirable in the art to have a particularsimple and economical process for grain 'refining zinc and mischmetalqcontaining magnesiumbase alloys, whereby a long lasting Igrainrefinement effect may be obt-ained without the need of additionalequipment or process steps.

The term long-lasting grain refinement effect as used herein means agrain refinement effect which persists throughout the period ofsettling, holding and casting withlout additional treatment in a largebatch of the molten magnesium-base alloy specified herein to be grainrefined in accordance with the present invention, for example, inbatches containing 600 to 4000 pounds of molten metal.

An object of Ithe present invention, therefore, is to l provide a novelprocess for grain refining magnesiumbase alloys containing zinc and rareearth metals.

Another object of the present invention lis to provide a simple andeconomical method of imparting a lasting grain refinement tomagnesiumabase alloys containing up 3,290,742 Patented Dec. 13, 1966 toabout 1.0 percent of rare ear-th metals and at least about 0.3 percentof zinc. A further object is to provide :such a method for grainrefining magnesium-base alloy ZE10 castings.

The appended two-figured lithograph, which will be discussed more `fullyhereinafter, 4of a photomicrograph taken at a 25 magnification of themagnesiumsbase alloy ZZElO shows in FIGURE 1 the alloy when treated inaccordance with the present invention and, as shown in FIGURE 2, whennot so-treated, thus illustrating the fine equiaxed grain structureobtainable in said alloy when processed by the -method of the presentinvention.

In general, the above and other objects and advantages have surprisingly-been found obtainable, in accordance with the present invention, bycasting or holding the magnesium-base alloy to be treated in the castingpot just prior to and during pouring at a temperature within the rangeof from about 1-190J F. to about 1240 F., and preferably between 1220F'. and 1240 F. The time and other temperatures, such as those employedin melting, alloying, .and `settling the alloy prior to being cast, donot affect the grain refining, provided the aforesaid temperatures inthe casting pot are observed just prior to and during pouring (casting)of the molten alloy. Higher temperatures than about 1240 F. should beavoided during casting, particularly when casting the ZE10 alloy. Forexample, the line carrying mol-ten metal from the cas-ting pot to themold shouldrnot raise the temperature of the molten alloy above 1240 F.

The alloy to be treated, in addition t-o containing up to 1 percent rareearth metal, such as, for example, by the addition of misch metal `(MM),and at least about 0.3 percent of zinc, should contain at least about0.001 percent of iron. In most cases, however, such an iron contentexists normally as an impurity in primary magnesium and :ordinarymagnesium alloys. Iron need not, there-fore, norm-ally be added to thealloy as a specific addition.

The method of the present invention, then, for Igrain refining amagnesium-base alloy as specified hereinbefore, comprises establishing amolten mass of said alloy at a temperature within the range of fromabout 1190 F. to about 1240 F. and, while maintaining said temperature,casting the metal in-to the desired configuration, thereafter allowingthe socast metal to solidify, thereby preparing fine grained castarticles. The present -method has the advantages of simplicity andeconomy in tha-t only the temperature of the molten metal need becontrolled during the normal casting sequence, thus requiring no Example1 To show how columnar grains and stringers are eliminated and fineequiaxed grains produced for a lasting grain refinement effect variouszinc and rare earth metalcontaining magnesium-base alloys operable inthe present novel process each having the composition indicated on TableI below were prepared in washed steel and clean graphite crucibles bymelting, alloying and flux refining said alloys at 1400 F. using acrushed fused flux having a composition comprising 50 percent magnesiumchloride, 25 percent potassium chloride, 20 percent barium chloride and5 percent calcium fluoride. The melts were then held for the periodsindicated in Table I, whereupon, each was sampled with a hot ladle whilethe melt was at a temperature within the range of from 1190 F. to 1400F. for grain size determination and composition analysis. In the smallermelts pounds), grain size was estimated from the fractures obtained from11/2 inch diameter by 8 inch long bars which had been cast into apermanent steel mold (P.M.). Samples using a wedge mold were alsoobtained from the same region of the melt for spectrographic analysis.Melts prepared for direct-chill continuous casting (commonly referred toas solidication casting) were prepared in 600 pound batches and werealso sampled for grain size and composition analysis. Other than themelts used for said direct-chill castings, which were cast in diametersas indicated in Table I, the grain size of which was determinedmetallographically, the grain size rating of the various melts wasdetermined by visual comparison with the naked eye of the aforesaidfracture samples and standard samples. A rating number was givenqualitatively related to the grain size and type was shown below:

Equiaxed Description of Columnar Number Grain Structure Number 01 1 02 203 3 04 4 05 Very coarse 5 y The structure was rated with an equiaxednumber only when there was a complete absence of columnar grains. If anycolumnar grains would appear the structure was rated with the respectivecolumnar number.

For comparison purposes a portion of the melts prepared above ascontrols were processed and sampled in the same manner except that thecasting temperatures employed were above 1240 F. The pertinent data andexternal vertical wall of the cylindrical cavity caused the metal tofreeze by spraying water on the mold wall as the pouring continued andthe resulting solidifying billet moved downward.

Part of the billet so formed was then cut into 2 inch slices and arepresentative sample slice selected for a grain sized and structuredetermination. The selected sample was then transversely cut intoquarters and one of the quarters cut into 4 segments perpendicularly tothe transverse cut. Said segments were designated 1 through 4 toward thecenter of the billet, and a portion of each segment was prepared formetallographic examination. A photomicrograph of the polished portionswas taken and is shown as FIGURE 1 on the appended lithograph.

For comparison, the above procedure was duplicated in every respectagain using the ZElO alloy except that the casting was done at atemperature above 1240 F., namely, at 1300 F. The photomicrographsobtained from the samples derived from this casting are shown as FIGURE2 of the appended lithograph.

An examination of the grain structure-of the segment portion designatednumber 1 in each iigure clearly shows the absence of stringers andcolumnar grains in the outer portions (close to the chilled surface) ofthe casting of FIGURE 1 cast in accordance with the present invention ascompared to the pronounced presence of same near the surface of the ZElOalloy of FIGURE 2, which was not cast in accordance with the presentinvention. lt should also be noted in segments 2 through 4 (whichproceed to the center of the casting) that the size of the equiaxedgrains in FIGURE 1 is markedly finer than that of the alloy of FIGURE 2,illustrating the exceptionally line equiaxed grains obtainable by themethod of the present invention.

f' results for these controls 1s also indicated 1n Table I. 3* It 1smanlfest that var1ous modlcations can be made TYPE I Holding History Ct.. Grain Structure Composition, Percent l as ing Sample Size ofMelt(Lb.) Type of Cast Temp.,

Tlme, Temp., F. Grain Width .of A1 Fe Mn MM 2 Zn Hrs. F. Rating StringerBand 000 contr01) 0" 11a.13.o.3 1g 1,300 .04 i'wide .003 .030 .0s .131.30 000 wam. 11o.3 1g 1.230 .03 None .00s .021 .07 .19 1.31000(contr01) 12 da. 13.0---. 18 1,300 1,300 .12 1V" wide .003 .035.o3 1. 000 12d1a.Do 14 1,300 1,237 .03 Nfm@ .002 .02s .000 100 (eontro11%d1a. 1 10 1,350 1,300 .o .037 .072 .10 1.43 100 1%"1113 x1 17 1,3501,220 .029 .071 .17 1. 40 100 e ntro1) 1%010 1 4 1,300 1,300 .020 .01.19 1.34 100 11/2Il d1a. 1 0 1,300 1,220 .020 .01. .1s 1.34

l Balance essentially of magnesium. 2 MM misch metal.

Example 11 To illustrate application of the grain-refining method of the'present invention with respect specifically to directchill casting, awell known casting technique, of the magnesium-based alloy ZElO whichhas the nominal composition of 1.3 percent zinc, 0.2 percent misch metal(MM), and 0.002 percent iron, the balance being essentially magnesium,about 600 pounds of the alloy was melted under a protective cover of theilux hereinbefore used and dened, whereupon it was continuously castusing a direct-chill mold to produce a 12 inch diameter billet 120inches in length. The direct-chilled mold comprised an open-toppedcylindrical cavity 12 inches in diameter having a hydraulicallydownwardly moving bottom part. The melt of ZElO alloy while at aternperature Within the range of from 1220c F. to 1240 F. wascontinuously poured into said cavity, whereupon, after a solidified headof metal had formed against the mold bottom, said bottom wasconcurrently and continuously moved downward as the melt was beingpoured into the mold. Water pipes located annularly around the 3 D.C.direct chilled.

4 P M. permanent mold.

in the process of the present invention without departing from thespirit or scope thereof and it is understood that I limit myself only asdeiined by the appended claims.

I claim:

1. A method of grain refining a magnesium-base alloy containing rareearth metals and zinc which comprises, lestablishing a molten mass ofthe alloy to be grain relined, casting said molten mass from atemperature within the range of from about 1190" F. to about 1240 F.,thereby imparting to the cast alloy a iine equiaxed grain structure, andcooling to solidify it.

2. The method of claim 1 wherein the casting temperature is Within therange of from about 1220 F. to about 1240 F.

3. The method of claim 1 wherein the magnesium-base alloy to be grainrefined contains at least about 0.001 percent by weight of iron.

4. The method of `claim 1 wherein the magnesiumbase alloy to be grainrefined contains by weight up to about 1 percent of rare earth metalsand at least about 0.3 percent of zinc.

5. The method of claim 1 wherein the magnesiumbase alloy to be grainrefined is the ASTM designated ZE10 alloy.

6. A method of grain rening a magnesium-base alloy containing up toabout 1.0 percent of misch metal and at least about 0.3 percent of zincand at least about 0.001 percent of iron which comprises, establishing amolten mass of the alloy to be grain rened, casting said molten massfrom a temperature within the range of from about 1190 F. to about 1240F., thereby imparting to the cast alloy a structure consisting of neequiaxed grains,x and solidifying the so-cast alloy by allowing it tocool.

7. The method of claim 6 wherein the casting temperature is within therange of from about 1220 F. to 1240 F.

8. A method of imparting a lasting ne equiaxed grain structure to adirect-chilled magnesium-base alloy casting of ZE10 which comprises,establishing a molten mass of said ZE10 alloy, continuously casting andsolidifying the molten alloy into a directly chilled mold from atemperature Within the range of from about 1190 F. to about 1240 F.,thereby to continuously produce a solidied casting of said ZE10 alloyhaving a lasting ne equiaxed grain structure throughout.

References Cited by the Examiner FOREIGN PATENTS 359,425 10/1931 GreatBritain.

OTHER REFERENCES Magnesium: 1st Ed., American Magnesium Corporation,1923, TN 799 M2 A6, page 115.

Magnesium: American Society for Metals, 1946, TN 799 M2 M18, page 73.

Journal of Metals: The Properties of Sand Cast Magnesiurn-Rare EarthAlloys, 1949, pp. 968-971.

I. SPENCER OVERHOLSER, Primary Examiner.

R. S. ANNEAR, Assistant Examiner.

1. A METHOD OF GRAIN REFINING A MAGNESIUM-BASE ALLOY CONTAINING RAREEARTH METALS AND ZINC WHICH COMPRISES ESTABLISHING A MOLTEN MASS OF THEALLOY TO BE GRAIN REFINED, CASTING SAID MOLTEN MASS FROM A TEMPERATUREWITHIN THE RANGE OF FROM ABOUT 1190*F. TO ABOUT 124*F., THEREBYIMPARTING TO THE CAST ALLOY A FINE EQUIAXED GRAIN STRUCTURE, AND COOLINGTO SOLIDIFY IT.